Hearing loss is the most common sensory deficit in the world, affecting almost 600 million people, and the most common congenital anomaly, affecting 2-6 per 1000 newborns. The most common type of hearing loss affects delicate mechanosensory and neural structures inside the inner ear, and is known as sensorineural hearing loss (SNHL). For the vast majority of people with SNHL the underlying cause is not known, because the inner ear cannot be biopsied at present without destroying hearing, and it is difficult to image cells (e.g., using techniques such as CT scanning and MRI) within the inner ear with sufficient resolution to establish diagnosis. Consequently, therapeutics are relatively crude and essentially limited to hearing aids and cochlear implants, which are prosthetics that are surgically inserted into the cochlea—the hearing part of the inner ear—to electrically stimulate residual neurons and bypass missing or nonfunctional cells. The performance of these devices varies widely, and the cellular origins of this variability are not well understood